1. Field of the Invention
This invention relates to apparatus for operating a gas discharge lamp, such as a fluorescent light, a mercury vapor lamp, a sodium lamp, or a metal halide lamp.
2. Related Applications
This application discloses subject matter which is related to United States Patent Applications Ser. No. 865,209, Filed Dec. 28, 1977; Ser. No. 940,435, Filed Sept. 7, 1978; and Ser. No. 968,372, filed Dec. 11, 1978; all by Francis Henry Gerhard and Gerald Allen Felper; all for "VARIABLE INTENSITY CONTROL APPARATUS FOR A GAS DISCHARGE LAMP".
3. Description of the Prior Art
Control circuits for gas discharge lamps are known which obviate the need for the usual heavy and expensive series ballast devices, corresponding to the inductor in this device. In such circuits, switching elements are provided to periodically reverse the direction of current through the lamp to reduce the deterioration or errosion of electrodes, and to ensure a high enough frequency of switching to reduce the requirement for the size of the ballast. Such circuits generally require two switching elements for each direction of the current.
Attempts have been made to fabricate the same type of circuit using only a single switching element to cause current reversal in the lamp. For example, the U.S. patent to D. B. Wijsboom, U.S. Pat. No. 3,906,302, is directed to such an arrangement and incorporates an inductor in parallel with the lamp, which lamp is in series with a switching device. Such a switching device is generally operated at relatively high frequencies, such as 20 KHz.
One problem has been that the fly back voltage during current reversal required to ignite the lamp when the circuit is first activated must be large enough to generate a sufficiently strong voltage gradient in the lamp to ionize the gas. This causes a large voltage to appear across the switching device which can damage the device during ignition, thereby limiting the reliability of the control circuit.
One solution to this problem is to use a step-up voltage transformer connected to a capacitive discharge device which provides sufficient voltage for a short period of time to ionize the lamp without requiring the flyback voltage of the control circuit to be large. However, this creates further problems because a step-up transformer must be connected in series with the lamp, and, after the lamp circuit has assumed normal operation, the large winding ratio of the transformer will cause significant current to flow in the primary winding with consequent power losses. This additional problem may be alleviated by opening up the primary winding after the lamp has ignited. However, this creates further problems because the secondary winding of the step-up transformer now acts as a second inductor in the lamp control circuit, impeding current flow through the lamp during flyback and further increasing the flyback voltage across the switching device, which may damage the switching device.
Another problem in the prior art has been that when a high pressure sodium lamp is used with the lamp control circuit, its resistance is well known to increase during the life of the lamp, which increases power consumption of the circuit, and decreases the efficiency of the lamp circuit.